1. Technical Field
The present invention relates to a fixing unit and an image forming apparatus with the fixing unit. In particular, the present invention relates a fixing unit attached to an image forming apparatus, such as a copier, a facsimile, a printer, etc., employing electrophotography, and an image forming method executed by the image forming apparatus.
2. Related Art
At present, various designs of image forming apparatuses, such as copiers, facsimiles, printers, etc., which employ electrophotography, have been developed and publicly known. An image formation process employed in the image forming apparatus is typically executed in the following steps. Specifically, an electrostatic latent image is initially formed on a surface of a photoconductive drum serving as an image bearer. The electrostatic latent image on the surface of the photoconductive drum is subsequently developed and rendered visible by developer, such as toner, etc. The developed image is subsequently transferred onto a recording medium (hereinafter, sometimes referred to as a sheet, a recording sheet, a recording member, a transfer member) by a transfer unit and is borne thereon. Subsequently, a fixing unit fixes the toner image borne on the recording sheet with pressure and heat, etc.
The fixing member and the pressing member are generally either opposed rollers or belts, respectively, or a combination of a roller and a belt, and are positioned adjacent to each other to form a fixing nip (hereinafter, simply referred to as a nip) therebetween, through which the recording medium bearing a toner image is conveyed.
For example, in a roller-type fixing unit, in which a fixing roller and a pressing roller are pressed against each other and form the nip therebetween, a recording medium bearing an unfixed toner image thereon is passed though the nip between the fixing roller and the pressing roller. One or both of these two rollers generally accommodates a heat source, such as a halogen heater, etc., and rotates while being heated and pressed against the other roller at the same time. Thus, the unfixed toner image is heated and pressed, and melts thereby being fixed on the recording medium at the same time.
In recent years, in accordance with a growing demand for greater energy efficiency while also shortening a waiting time needed for heating a fixing unit (e.g., a warming-up time, a first time to print, etc.,), a so-called on-demand fixing unit is widely adopted, in which an endless belt unit composed of a belt or a thin film and the like is employed instead of a roller unit including a fixing roller or the like. Such an arrangement serves to reduce heat capacity of the fixing unit, thereby improving efficiency of heat transfer to the recording medium while significantly shortening waiting time
As an example of this kind of known fixing unit, JP-2008-158482-A discloses a fixing system in which a fixed member (e.g., an opposed member) is pressed against an inner circumferential surface of a belt unit and a pressure roller (e.g., a pressing roller) via the fixed belt unit while sliding along the belt unit and forming a fixing nip therewith. A recording medium is conveyed to the nip to fix a toner image on the recording medium. A heat transfer member (e.g., a heating member) is also provided in the system to support the fixing belt unit by either approaching or contacting the inner circumferential surface of the fixing belt unit at a prescribed position except for the nip.
When a relatively heavy load is applied to the nip to obtain a given amount of nip pressure in the fixing unit, a torque increases in the nip thereby possibly raising a problem. Such a problem can be solved if a contact area (i.e., a friction sliding area) between the fixing belt unit and the heat transfer member is reduced, thereby minimizing the torque therein. To reduce the contact area and accordingly suppress increasing in torque in a fixing unit, JP-2012-145708-A discloses prescribed grooves formed in a surface not heated by a heater outside of a heated area of the heating device and extended perpendicular to a direction in which a recording medium passes through a fixing nip N.
JP-2008-275755-A also discloses a heating device, in which a film unit (i.e., a belt member) and a film unit holder that holds the film unit are provided. The film unit holder includes a recessed portion in its surface contacting the inner circumferential surface of the film unit to reduce a contact area in which the film unit and the film unit holder contact each other. Hence, by reducing the contact area, torque required in the contact area can be reduced.
However, reducing the contact area of the heat transfer member and the inner circumferential surface of the fixing belt unit widens a gap formed between the heat transfer member and the inner circumferential surface of the fixing belt unit resulting in unstable movement of the fixing belt as a problem.
Further, although the torque can be reduced by the conventional systems as disclosed in the JP-2012-145708-A and JP-2008-275755-A, the gap between the heat transfer member and the inner circumferential surface of the fixing belt unit cannot be reduced, and accordingly the problem of unstable movement of the fixing belt remains unsolved.